Circuit boards are generally insulative flat sheets, rigid or flexible, having a pattern of electrically conductive paths to which wires and electronic components may be connected. Terms such as printed circuit board and printed wiring board are frequently used for these devices. The pattern of conductors on the board surface typically consists of a multiplicity of generally straight line segments of variable width and direction, with pads of greater diameter at some intermediate position in the path or terminating a path. There may be holes through the centers of these pads and through the board through which wires or leads of electronic components are passed for connection to the pads.
Although techniques for manufacturing circuit boards are highly advanced, a number of possible defects prevent the manufacturing yield from being less than 100%. Thus it is desirable, and for some purposes imperative, to inspect each board for defects which could lead to inoperativeness of the electronic component or circuit of which the board is a part. Typical defects in the conductive pattern relate to line widths, line spacings, cracks or voids, rough edges, shorts or merging between conductive paths or pads, and hole irregularities. Such inspection may be done visually with microscopes and machines have been developed for automatic inspection of circuit boards. Wiring board inspection machines frequently use one or more electrooptical cameras with extraordinarily high resolution. High density boards have line widths and spacings as low as 0.003 inch. For such boards the resolution of the optical detector in the camera must be in the order of 0.001 to 0.0003 inch to adequately inspect the board.
Optical detectors typically employ semiconductor linear arrays of up to 2,048 detectors or photo diodes in a camera, each individual detector in the array measuring the light intensity to which it is exposed. A typical detector is 0.001 inch square. With the optics of a typical camera magnified by a ratio of 3:1, an instantaneous field of view at the wiring board of approximately 0.0003 by 0.3 inch can be obtained.
The camera and the wiring board are physically moved with respect to each other in order to scan the instantaneous field of view of the camera over the entire circuit board. A circuit board inspection machine of the type mentioned above is disclosed in U.S. Pat. No. 4,185,298.
Several different methods for illumination of the circuit board surface to be inspected have been devised. One process involves illuminating the inspected surface of the board from the side on which the detector is located. Another technique employs back lighting wherein the illumination passes through the board from the side opposite the detector.
In the front lighting technique the camera measures the light reflected from the wiring board surface. The differences between conductor reflectivity and nonconductor reflectivity is a crucial parameter. Differences in design and in the condition of the circuit board can result in non-detection of defects as well as false alarms where no defect exists. An example of the condition variations include the fact that conductive paths may be claen and bright or variably tarnished and dull. Further, a number of different materials may comprise the exposed surface of the conductive path. Also the surface contours of the conductive paths are variable, specifically where wiring boards are tinned the top surface may be convex thereby producing specular reflections of substantial variability. Thus reflective systems are not effective to determine circuit path width, nor do thay readily detect underetch, which is a rough bridge of conductive material between paths. Also a thin flexible board can, in effect, fool the reflective type system because a circuit path on the non-inspected side could be reflected through and spuriously indicate the presence of circuitry that does not exist on the surface of interest.
In systems using back lighting, the detectors in the camera measure light transmitted completely through the wiring board. All conductors are opaque and appear dark regardless of their surface condition so some of the problems associated with front lighting are avoided. A significant limitation associated with the back lighting technique is that it is generally usable only with boards having circuitry on one side. If the board has conductive paths on both sides, back lighting will cause dark areas or shadows to be detected by the camera whether they are on the inspected side or the back side of the board. Thus the information received by the camera would be an intermingling of the conductive paths on both sides and would prevent such inspection apparatus from producing useful results. Additionally, many boards have substantially an entire surface plated with a conductor which would be fully opaque to back lighting and therefore this kind of system would not be appropriate for a board of this type.
The problem common to most prior circuit board inspection systems is accurate and reliable discrimination between the conductive areas and the substrate.